Field of the Invention:
The invention relates to a fiber-optic apparatus for receiving emitted radiation from a diode laser.
A diode laser is normally composed of a plurality of so-called diode laser bars. Each diode laser bar in this case comprises a multiplicity of emitters. The emitters are narrow, rectangular semiconductor elements which are themselves composed of a multiplicity of semiconductor surfaces which are arranged closely adjacent alongside one another. The narrow faces of the semiconductor surfaces are arranged alongside one another in a row.
A diode laser bar produces a narrow, approximately rectangular, laser beam whose beam characteristics in the longitudinal direction differ from the beam characteristics on the axis at right angles to this. Owing to these asymmetrical beam characteristics and the beam combination which is normally required of a plurality of diode laser bars in order to increase the power of the diode laser, its design requires beamforming, that is to say beam symmetry and beam superimposition, of the emitted radiation. This applies in particular to high-power diode lasers in which the laser beam is intended to be carried by way of optical fibers.
Various concepts are described in the prior art for the process of making the beam symmetrical. The beam reshaping can be carried out reflectively with the aid of microlayers, refractively by means of microprisms, or else diffractively. Free-beam concepts which operate with micromirrors or microprisms allow the original beam quality of the diode laser bar to be largely maintained. Concepts such as these, (cf., for example, German patent DE 195 00 513 C1) require very exact manufacturing processes, however, and therefore involve a large amount of technical effort and complex adjustment of the components. The costs of a device such as this for beam reshaping or beam forming are therefore relatively high.
A further possibility for beam reshaping is to inject the laser beam emerging from the individual emitters in each case into an optical fiber associated with it, and for the optical fibers subsequently to be reoriented from a linear arrangement to a circular arrangement. Beam reshaping such as this can be achieved considerably more cost-effectively with the aid of fiber bundles (cf., for example, European patent EP 0 486 175 B1). However, the concepts which have been used until now in each case provide one optical fiber per emitter, of which a typical diode laser bar has approximately 19, with a lateral extent in the longitudinal direction of 150 μm in each case. As a result of the highly asymmetric emission of an individual emitter with an extent of about 1 μm×150 μm and a divergence angle of about 30°×5° (half angle), the use of round optical fibers, whose diameter must correspond at least to the lateral extent of an emitter, unavoidably leads to high losses with regard to the beam quality at the optical fiber outlet. The beam quality in the vertical extent in the direction of the narrow face, the fast axis, is decreased by a factor which corresponds to the ratio of the diameter of the optical fibers and the vertical extent of the fast axis of the emitter. European patent EP 0 486 175 B1 therefore also discloses the use of rectangular optical fibers, whose lateral extent is matched to the length of the emitter. In one embodiment, their extent at right angles thereto is between 30 and 50 μm.
As an alternative to this, international PCT publication WO 97/42533 A1 discloses an assembly in which round optical fibers are provided. These are arranged, at their free end facing the emitter, in a wedge shape, in each case with a rectangular inlet surface. Individual fibers with cross-sectional shapes such as these can be produced, however, only with a large amount of manufacturing effort.
U.S. Pat. No. 5,546,487 discloses an assembly in which the laser beam which emerges from an emitter (1 μm×100 μm) which comprises a multiplicity of facets (1 μm×5 μm) is injected into a multiplicity of optical fibers with a rectangular cross section, which has an extent at right angles to the lateral direction which is approximately 2.5 to 15 times greater than that of the emitter. The laser beam emerging from an emitter is collimated in this direction (vertically) with the aid of a cylindrical lens, which is arranged between the diode laser bar and the optical fibers, to a beam size which is smaller than the vertical extent in the optical fibers, such that it illuminates only a portion of the inlet surface of the optical fibers. This results in a deterioration in the beam quality on the fast axis.
German patent DE 10 2004 006 932 B3 describes a diode laser in which the emitters have an associated device for beam forming and the guidance of the laser beam emerging from it. In this case, the emitters are arranged in a row alongside one another in the direction of their longitudinal axis (slow axis). This device for beam forming and beam guidance has a multiplicity of optical fibers, which are arranged alongside one another in a row. The laser beam of each emitter is injected into these optical fibers such that each emitter is associated with a multiplicity of optical fibers. In order to avoid radiation losses resulting from empty spaces between the individual circular cross sections, the fiber inlet cross sections, in this case in rectangular form, are matched to the geometry of the emitter. In this case, the fibers are arranged alongside one another in a row, touching, and without any empty spaces. The rectangular cross section of the fiber inlet surfaces is achieved by means of a hot-pressing method, with the individual fibers which are arranged in a row at the same time being integrally connected to one another.
All the known concepts have the common feature that the laser beam which emerges from the diode laser bar that has a width of about 10 mm (longitudinal direction of the diode laser bar, slow axis) is optically segmented and geometrically reshaped. However, in order to comply with the requirement for beam quality, this necessarily leads to highly complex solutions which can be technically implemented only with difficulty, and are therefore very expensive.
The optical fibers are in this case arranged with their ends alongside one another in a row. An arrangement such as this can be implemented technically only with a large amount of effort. The described reshaping of the fibers to form a rectangular cross section is likewise technically complex and difficult to manufacture. Furthermore, the described fiber-optic solutions are mechanically highly sensitive and must be handled extremely carefully.
The prior art is in this case based on the linear arrangement of round fibers leading to losses at the beam inlet because of empty spaces between the circular fiber cross sections. Embodiments are therefore described as being particularly advantageous in which the optical fibers have inlet cross sections which are matched to the rectangular geometry of the laser beam which emerges from the diode laser bars.